This invention is directed to a MOCVD process of fabricating a solid-state UV detector whose long wavelength cut-off can be set between 220 and 360 nm for flame sensing and other applications. The detector is base on the aluminum gallium nitride (AlGaN) material system.
In the prior art it is known to have photocathodes and photomultiplier-tubes (PMT's) which sense ultraviolet (UV) radiation. The PMT's are costly, large size and fragile, and they require high voltage. In addition the long wavelength cut-off of these detectors is not adjustable and they respond to wavelengths longer than 300 nm. Filters can be used to reject wavelengths longer than 300 nm but this adds mass and cost. The solid-state AlGaN detector of this invention is an ideal replacement for the PMT's having low mass, reliability, low cost and has a sharp cutoff wavelength for UV detection.
In the prior art certain UV detectors of Al.sub.x Ga.sub.1-x P have appeared in the literature. Two of these articles are by the same authors A. R. Annoeva et al, "Photoelectric Effect in Variable-Gap Ga.sub.1-x Al.sub.x P Surface-Barrier Structures", Sov. Phys. Semicond. 15(1) January 1981, P64-66 and "Ultra- violet Photodetector Based on a Variable-Gap Ga.sub.1-x Al.sub.x P.sub.x (x.sub.s =0.5+0.1) Surface-Barrier Structure", Sov. Phys. Semicond. 15(6) June 1981, P646-7. These prior art AlGaP devices were grown by liquid phase epitaxy (LPE). A third article data Feb. 81 is by Donald L. Smith and Richard H. Bruce, entitled "Growth of Aluminum Gallium Nitride Films for Electro-optic Device Applications" and is an unrestricted report to the Office of Naval Research under a Contract No. N00014-77-C-0492. In the prior art an article by Khan et al, "Properties of Ion Implantation of Al.sub.x Ga.sub.1-x N Epitaxial Single Crystal Films Prepared by Low Pressure Metal Organic Chemical Vapor Deposition", Appl. Physics Letters, September 1983 teaches one method by which Al.sub.x Ga.sub.1-x N has been grown on a sapphire substrate for use as an optical device in the UV region of the spectrum.
The present invention provides an improved method of growing a AlGaN sensor for ultraviolet which solves a problem of detecting UV radiation against a hot refractory background or solar radiation. Thus this detector can sense UV radiation of a flame in a furnace with a hot refractory background, for example, responding only to the UV and not to radiation of other wavelength emanating from the hot furnace interior. This UV-detector is based on interband absorptions of incoming radiation in the AlGaN material system. The detector does not require any additional filter as the intrinsic absorption cutoff in the semiconductor acts as a filter. The method includes first growing a layer of AlN on the sapphire substrate and then the AlGaN layer upon which a photodetector structure is fabricated.